\chapter{Current Technologies}
According to the introduction, LBSs are not used that much. Particularly emergency services make high demands on LBS infrastructure providers concerning accuracy results, interoperability and scalability. In this chapter, first of all today's positioning methods in the context of mobile networks are presented in order to discuss state of the art solutions.

\section{Positioning Methods}
One crucial point when providing LBSs is the granularity of the accuracy determined by the \textit{Location Determination Technology} (LDT). In any positioning technology there are possible inaccuracies and a probability of error. For estimating the dimension of positioning errors the following accuracy measures are commonly used:

\begin{itemize}
  \item \textit{Positioning Error} (PE)
  \item \textit{Circular Error Probability} (CEP)
  \item \textit{Cumulative Distribution Function} (CDF)
  \item \textit{Root Mean Square Error} (RMS)
\end{itemize}

Further information about the calculation can be found in Database Correlation Method for Multi-System Location \citep{Kempii2005}.

\newpage
\noindent
LDTs can be classified as \textit{terminal-based} or \textit{network-based} methods.

\subsection{Terminal-Based Positioning Methods}
These methods are characterised by handsets calculating their positions themselves. Two terminal-based LDTs are presented below.

\subsubsection{A-GPS}
 One way of terminal-based positioning is based on \textit{satellite-based} positioning systems. GPS is one of such systems among others. One crucial disadvantage of GPS is the \textit{Time to First Fix} (TTFF) delay. When calculating GPS positions the GPS receiver firstly looks up the positions of the satellites which causes the TTFF delay on the delivery of the result. \cite{Andersson2001} notes, common TTFF values are in the range of 20 to 45 seconds. Relating to LBSs, such a delay limits the LBS usability because end-users have to wait this amount of time to get the nearest restaurants for instance.

The TTFF delay can be decreased by using the network to provide complementary data that the MT can use for the calculation. This technology is called \textit{Assisted-GPS} (A-GPS), visualised in figure 3.1, and is the only advanced positioning method. According to \citet[p. 25]{Hjelm2002}, if the information about the positions of the satellites can be provided to the MT before it starts searching for them, it will save a lot of time and thus battery life. The point is, that the MT firstly knows the cell in which it is located in the mobile network and secondly because of the synchronisation with the BS the precise time signal. This information can be used for the calculation of the satellite orbits so that the antenna can point at approximately the right places, get a fix, and calculate the position faster, as \cite{Hjelm2002} mentions.

\begin{figure}[h]
		\begin{left}
	    	\includegraphics[height=4.5cm]{graphics/agps}
				\caption[Overview A-GPS]{Overview A-GPS \citep[p. 52]{GSMAssociation2003}.}
			\label{Overview A-GPS}
		\end{left}
\end{figure}

\newpage
\noindent
There are two ways of doing A-GPS \citep{Hjelm2002}:
\begin{itemize}
  \item \textit{Functionality in MT}\\
  This mode requires, that the A-GPS receiver stores mappings containing the \textit{cell-id} and the relating GPS position in the memory of the MT, when the cell-id is used as a base for calculating the satellite positions. By means of that mapping and the exact time the A-GPS receiver can calculate the approximate position of the satellites. However, one problem of this approach is the distribution of the mappings. In fact it might be a satisfied solution for a combination of a few PLMNSs. But on the other hand, when thinking about roaming and interoperability between different operators, this approach is impossible because of the enormous required amount of mapping data and the ongoing infrastructure changes.
  \item \textit{Transmission of satellite data over the network}\\
  Thereby, the satellite data is downloaded over the mobile network. The GPS receiver in the handset is not a full GPS receiver, and the data (e.g. GPS location of the cell) needed to calculate the position is sent every time. This situation means a higher load on the network, and enables the operator to keep the cell structure confidential.
\end{itemize}

A-GPS interoperates well with UMTS \citep{Hjelm2002}. LDTs, based on GPS, enable three dimensional positioning results. Some specific applications, such as for rescue operations in the mountains this feature might be of value because rescuers know the approximate altitude of the injured person. On the other hand, the most significant limitation of GPS has always been that it requires a clear view of the sky, as \citet[p. 256]{Andersson2001} states. Therefore this technology works badly indoors. Further information about satellite-based positioning technologies can be found in Location-Based Services \citet[chap. 7]{SchillerVoisard2004}.

\subsubsection{E-OTD}
Relating to \citet[p. 258]{Hjelm2002}, a concern with GPS is that it requires new hardware in the receiver which is always something that device manufacturers are reluctant to include. Hence a software-based solution is preferable and is more cost-effective. \textit{Enhanced Observed Time Difference} (E-OTD) calculates the time difference that it takes to receive data between different base stations and the MT and estimates the position based on that information. In practice the signals are sent on the \textit{control channel}. Additional information like absolute GPS references of the \textit{Base Stations} (BSs), identity signals, and timing information are required to calculate the position of the MT.

\begin{figure}[h]
		\begin{left}
	    	\includegraphics[height=6cm]{graphics/otd}
			\caption[Overview E-OTD]{Overview E-OTD \citep[p. 51]{GSMAssociation2003}.}
			\label{Overview E-OTD}
		\end{left}
\end{figure}

As illustrated in figure 3.2, this system requires at least three BTSs to be in range of the mobile terminal to calculate the position of the MT based on triangulation. As \cite{Andersson2001} notes, in order for this process to work, an overlay network of \textit{Location Measurement Units} (LMU) needs to be deployed to provide an accurate timing source for the measurements. The E-OTD capable MT calculates the time difference between the signals from the measured BTSs. This time difference is then a measure for the distance between each of them, in order to finally calculate the relative position by using triangulation. As mentioned above, the absolute reference BSs coordinates have to be known to calculate the absolute position of the MT afterwards. A quite similar method has been standardised for 3G networks, also known as \textit{Observed Time Difference Of Arrival} (OTDOA).

\subsection{Network-Based Positioning Methods}
The terminal-based LDTs either demand for additional hardware or software features to calculate the position on MTs. \textit{Network-Based Positioning Methods} do not require neither any hardware nor any software in MTs because a back-end component,  often known as MPC, in the PLMN is responsible for calculating the position. Further information of these LDTs can be found in the next section.

\subsubsection{Cell-Id}
One side-effect of PLMNs is that a simple determination of the user's location is possible because of their cellular architecture. The cell-id method is the most elementary network-based LDT. It uses the identification of a cell to roughly determine the position of the mobile participant. Hence the granularity of this method depends on the cell size. One big advantage of this method is, that legacy MTs can be positioned.

\begin{figure}[h]
		\begin{left}
	    	\includegraphics[height=4.5cm]{graphics/ci}
			\caption[Overview Cell-Id]{Overview Cell-Id \citep[p. 49]{GSMAssociation2003}.}
			\label{Overview Cell-Id}
		\end{left}
\end{figure}

As illustrated in figure 3.3, the PLMN is divided into cells. Each of these cells contain a BTS which houses the radio transceivers in order to handle the radio-link protocol for data transmission with the MT. \citet[p. 163]{Main2003} states, that PLMNs must manage mobile stations that may be constantly changing locations regardless whether the MT is in any use or on standby. When the MT crosses cell borders, a so called \textit{Handover} takes place.

The \textit{Mobile Switching Center} (MSC) usually manages handovers based on:

\begin{itemize}
  \item Low signal strength (\(<\) Limit)
  \item High error occurrence (\(>\) Limit)
  \item Maximal distance between MT and BTS
\end{itemize}

There are two types of handovers. First of all the \textit{Network-Handovers} where the MSC decides based upon the three aforementioned facts. The MSC monitors the adjacent cells and determines which cell provides the best service. Secondly there are \textit{Mobile-Assisted-Handovers}, where the MT delivers context measurements (signal strength to neighbouring cells, etc.) to the MSC for making the decision.

Basically two back-end components are involved when a handover occurs. Besides handling subscriber's profiles the HLR keeps track of the location of all \textit{home} subscribers. So when a handover is processed the serving cell is updated in the HLR. On the other hand, if the user is roaming the \textit{Visitor Location Register} (VLR) observes all visiting subscribers served by a MSC that is not in their home area. In effect the VLR saves the \textit{Mobile Station Integrated Services Digital Network number} (MS-ISDN) combined with the serving cell. Then the VLR exchanges this information with the subscriber's HLR. Thus the subscriber's HLR will know the NO and the cell, if the mobile user is not logged in its home network. Worth mentioning thereby is, that this scenario is further complicated since \textit{Mobile Number Portability} (MNP) has been introduced. MNP permits subscribers to change the provider and keeping the same MS-ISDN.

In conclusion every time when the location is requested the subscriber's HLR is queried even though the mobile user is roaming. Further information can be found in Location Based Services \citep{GSMAssociation2003}.

\subsubsection{Cell-Id + TA}
For improving the cell-id LDT the \textit{Timing Advance} (TA) value can be used, as illustrated in figure 3.4. The TA is used to handle the time slot management correctly.  If the MT transfers data to the BTS, the MT will pay attention to the TA because it determines how much time the data has to be send earlier in order to arrive in time at the BTS, in the time slot allocated for the MT. As \citet[p. 31]{Hjelm2002} notes, the resolution is one GSM bit, which has the duration of 3.69 microseconds. Because this value is a measure of the round trip from the BTS to the MT or reverse, the resolution is 1.85 microseconds, which equals 550 meters at the speed of light. It improves accuracy in some cases.

\begin{figure}[h]
		\begin{left}
	    	\includegraphics[height=4.5cm]{graphics/ta}
			\caption[Overview Cell-Id + TA]{Overview Cell-Id + TA \citep[p. 50]{GSMAssociation2003}.}
			\label{Overview Cell-Id + TA}
		\end{left}
\end{figure}

When combining cell-id with TA, it is possible to determine how far away from the BTS in the cell the user is located.

\subsubsection{Cell-Id + RSCP}
When a MT connects to the BTS, it will measure the effect needed to connect to the BTS near it, not only that to which it is connected, but also those that have lower field strength and are therefore farther away, as \citet[p. 32]{Hjelm2002} notes. The \textit{Received Signal Code Power} (RSCP), also known as \textit{RxLev}, can be used therefore to measure the distance from the BTS to the MT. ``The power measured at the handset is the same as the power transmitted from the base station plus the gain in the antenna of the base station, minus the loss during the transmission path, plus the gain in the antenna at the mobile station'' \citep{Hjelm2002}. According to \citeauthor{Hjelm2002}, this fact can be used to figure out the distance because the path loss is related to the distance.

\subsubsection{UL-TOA}
\textit{Uplink-Time Of Arrival} calculates the position based on the \textit{uplink time} from the MT to the BTS. The propagation delay from the MT to the BTS, as presented in figure 3.5, is calculated by the current and adjacent BTS and based on this information the position is estimated by triangulation.

\begin{figure}[h]
		\begin{left}
	    	\includegraphics[height=5.5cm]{graphics/ultoa}
			\caption[Overview UL-TOA]{Overview UL-TOA \citep[p. 259]{Andersson2001}.}
			\label{Overview UL-TOA}
		\end{left}
\end{figure}

This method requires in addition to LMUs synchronised BTSs. The synchronisation of BTSs is mostly established by means of GPS receivers. Because UL-TOA relies on the network, it works with any kind of MT. Worth mentioning is that this method was adapted for 3G environments, also known as \textit{Uplink-Time Difference of Arrival} (U-TDOA).

\subsubsection{AOA}
\textit{Angle Of Arrival} demands that the BTSs are equipped with special antenna arrays to determine the angle of the signal arriving from the MT. Measurements from two different BTSs are enough for locating the MT. The aforementioned antenna arrays are normally not included in standard BTSs, and therefore cause additional costs for operators. Worth mentioning is the fact, that the positioning accuracy of the method often decreases in non line-of-sight (NLOS) conditions, like \citet{Kempii2005} notes.

\subsubsection{DCM}
\textit{Database Correlation Method} (DCM) has not been standardised yet, and is a general location method for cellular or WLAN networks. As \citet{Kempii2005} states, the idea is to create a database of reference fingerprints for the area of interest. The reference fingerprint is a recorded measurement \textit{sample} from a specific location somewhere in the area of interest. This sample contains all context information, which can be measured at this specific location:

\begin{itemize}
  \item \textit{Current and neighbouring cell information}
  \item \textit{Current and neighbouring signal strength}
\end{itemize}

When positioning MTs the current available sample data are compared with all reference fingerprints. The location with the best matching reference fingerprint is returned.

One disadvantage of this method is, that fingerprints of the whole area of interest has to be known in order to apply DCM. This can be compensated by means of \textit{Propagation Models}, which are used to estimate the propagation of radio waves in a particular area, and subsequently to create a reference fingerprints database artificially. More information in this matter can be found in Database Correlation Method for Multi-System Location \citep{Kempii2005}.

\subsubsection{Hybrid Methods}
Today's LBS vendors often describe its LDTs as \textit{hybrid} methods. The previously listed LDTs can be combined to improve the accuracy and to achieve a better performance in different environments. These combination of various LDTs are also known as hybrid methods. In urban areas a more enhanced LDT is applied whereas in rural areas only an elementary LDT can be used to save costs.	

\subsection{Comparison Positioning Methods}
Table \ref{comparisonPosMethods} compares the above mentioned LDTs in detail. Measurements were conducted in GSM environments. The figures were fetched from Database Correlation Method for Multi-System Location \citep{Kempii2005}, Location Based Service \citep{GSMAssociation2003} and GPRS and 3G Wireless Applications \citep{Andersson2001}.

\begin{table}[htbp]
\caption{Comparison positioning methods.}
\begin{tabular}{llll}
%
\\
\textbf{Technology}&
\textbf{Accuracy}&
\textbf{Advantages}&
\textbf{Disadvantages} & \hline
%
A-GPS &
10 to 100m &
High &
High network &
%
 &
 &
accuracy &
investments; new &
%
 &
 &
 &
handsets required &
%
 &
 &
 &
with expensive &
%
 &
 &
 &
chip-sets and high &
%
 &
 &
 &
battery consumption & 
%
 &
 &
 &
 &
%
E-OTD &
50 to 150m &
Highest &
High network &
%
 &
 &
accuracy of &
investments &
%
 &
 &
non-GPS &
required; E-OTD &
%
 &
 &
solutions &
enabled handset &
%
 &
 &
 &
required &
%
 &
 &
 &
 &
%
Cell-Id &
Depends on & 
Legacy MT &
Low accuracy &
%
 &
cell size: &
support; &
 &
%
 &
urban &
Cheapest of &
 &
%
 &
500m-5km &
all techno- &
 &
%
 &
rural &
logies for &
 &
%
 &
1km-35km &
operators &
 &
%
 &
 &
 &
 &
%
Cell-Id + TA &
Up to 50\%\ &
Better &
Low to moderate &
%
Cell-Id + RSCP &
improvement &
accuracy &
accuracy &
%
 &
over basic &
especially in &
 &
%
 &
Cell-id &
rural areas: &
 &
%
 &
 &
legacy MT &
 &
%
 &
 &
support &
 &
%
 &
 &
 &
 &
%
UL-TOA &
50 to 150m &
Legacy MT &
Highest network &  
%
 &
 &
support &
investments &
%
 &
 &
 &
required because &
%
 &
 &
 &
of BTS synchronisation &
%
 &
 &
 &
 &
%
DCM &
Area of interest &
Legacy MT &
Pre-Scanning of area &
%
 &
previously scanned &
support &
of interest or &
%
 &
urban &
 &
non-trivial &
%
 &
\~\ 50m &
 &
implementation &
%
 &
rural &
 &
because of propagation- &
%
 &
\~\ 80m &
 &
models & \hline
%
\end{tabular}
\label{comparisonPosMethods}
\end{table}

\subsection{Discussion Positioning Methods}
Analysing the aforementioned LDTs, the more enhanced methods use triangulation for calculating the position of MTs. The surrounding environment, particularly the \textit{radio wave propagation}, has an effect on the accuracy of these LDTs. Because of \textit{reflection}, \textit{diffraction}, \textit{scattering} and \textit{absorption} radio waves travel different paths from the transmitter to the receiver, which is known as \textit{Multipath Propagation}, as \citet{Kempii2005} publishes. Bringing this phenomena in the context of LBSs the multipath propagation has impact on the calculated time difference of the signal propagation between the BTS and the MT and hence on the user position.

According to the \citet[p.75]{GSMAssociation2003}, positioning technologies can be divided based on the achieved accuracy of the method in three levels, as illustrated in table \ref{positioningMethods}:

\begin{enumerate}
  \item \textit{Basic} Level\\
  Location of all handset including legacy devices (Cell-Id + TA).
  \item \textit{Enhanced} Level \\
  Location of all new handsets with improved accuracy and reasonable costs (UL-TOA and E-OTD).
  \item \textit{Extended} Level\\
  Location of new handsets with high accuracy and higher costs with customer choice (A-GPS).
\end{enumerate}

Other factors to consider as the \cite{GSMAssociation2003} states, are the complexity of the system and the investments needed on the network side and possibly in handsets.

\begin{table}[h]
\caption{Positioning methods.}
\begin{tabular}{lll}
%
\\
\textbf{Level}&
\textbf{Method}&
\textbf{Handset dependence}\\ \hline
%
Basic &
CI, CI+TA &
No &
%
Enhanced &
E-OTD, TOA &
Yes/No &
%
Advanced &
A-GPS &
Yes & \hline
%
\end{tabular}
\label{positioningMethods}
\end{table}

An interesting fact is, that the \citeauthor{GSMAssociation2003} preferred \textit{handset-assisted} modes over \textit{handset-based} modes \citep[p. 28]{GSMAssociation2003}. The handset-based mode calculates the user position in the handset and subsequently sends this back to the network whereas in the handset-assisted mode the MT makes only measurements and reports these back to the network, which calculates the handset position.

Analysing this fact, security concerns play an important role. Handset-assisted modes do not store the location in the terminal, thus preventing the access to subscribers' location data. Furthermore the \citet{GSMAssociation2003} states, that this presents a privacy threat to the subscriber. On the other hand, infrastructure providers and terminal suppliers have prioritised handset-based modes, over handset-assisted modes.

Obviously it looks like that the involved parties have not come to an agreement regarding this matter. Furthermore the \citet{GSMAssociation2003} states, that MT manufacturers should ensure that any location data stored on the terminal as a result of legitimate location service is not accessible by any application that has not been properly authenticated.

\section{Solutions}
A couple of LDTs exist in the context of LBSs. This section presents cutting-edge LBS infrastructures which mostly combine the previously mentioned LDTs to provide positioning. The fundamental idea of these solutions is to provide standardised interfaces either for mobile operators or for service providers.

\subsection{Ericsson}
As a supporter of the LIF, Ericsson provides in its \textit{Mobile Positioning System} (MPS) infrastructure a solution which permits widespread deployment of LBSs with roaming capabilities. Third party applications communicate with the \textit{Gateway Mobile Position Center} (GMPC) using the XML based MLP protocol to get positions of MTs. Particularly for third party applications Ericsson abstracted the MLP protocol and offers a Java based API, \textit{JMPP} API. On the other hand this system is designed for ``critical'' applications such as emergency services. Hence internal applications interact with the GMPC over an SS7-based protocol or via TCP/IP, which is defined in the Parlay API.

Ericsson's MPS is compliant in 2G and 3G environments. According to \citet{Ericsson2006}, Mobilaris and LocatioNet are the LBS middleware supplier partners of \citeauthor{Ericsson2006} to verify the successful integration into several mobile networks. This system  is used in the US, Finland, Ireland, France, Spain, Greece, Norway, Turkey, Serbia, China, Taiwan, Australia, etc. \citep{Ericsson2006}.

\subsection{Openwave}
A bit different from Ericsson, Openwave has developed its own location infrastructure \textit{Location Manager}. Openwave categorises its product portfolio into a ``Commercial Edition'' and an ``Emergency Edition''. As Ericsson, Openwave is a member of the LIF and therefore offers similar services. Openwave Location Studio supports WAP and is compliant with the OpenLS interface. Besides classical positioning services Location Studio supports the integration of third party geo-spatial servers and offers specific functionality for privacy, policy, personalisation and enforcement.

Openwave's Location Manager is compliant in 2G and 3G environments and used in the US, Japan, UK, etc. \citep{Openwave2006}.

\subsection{Qualcomm}
Another provider and also member of the LIF is Qualcomm. Its \textit{gpsOne} has been the first deployed A-GPS technology. Qualcomm has developed specific A-GPS chipsets, which are directly integrated into the \textit{Mobile Station Modem} (MSM) eliminating the need for separate hardware components in order to provide cost-effective, space-efficient A-GPS on MTs. 

gpsOne is designed for any kind of air interface including 2G and 3G technologies. This technology has been successfully deployed in the US, Canada, Central America, South America, Japan, South Korea, China, the Middle East and Thailand \citep{Qualcomm2006}.

\subsection{Cambridge Positioning System}
CPS is a further supporter of the LIF and offers in its CPS \textit{Matrix} product suite enhanced LDT in the context of 2G and 3G. The fundamental services, known as Matrix, uses everyday network synchronisation signaling to determine locations. Furthermore CPS offers GPS based solutions combined with Matrix, known as E-GPS \citep{Cps2006}.

\subsection{True Position}
True Position's \textit{True North Managed Location Services} follows the guidelines of the LIF and offer any kind of LBSs. Furthermore their infrastructure plays an important role in the ongoing implementation of the US national wireless E-911 system and in the national E-112 system for emergency services in Europe \citep{TruePosition2006}.

\subsection{Comparison Solution}
Table \ref{comparisonSolutions} presents each infrastructure provider mentioned before in addition with the LDTs supported.

\begin{table}[htbp]
\caption{Comparison solutions regarding LDTs supported.}
\begin{tabular}{ll}
%
\\
\textbf{Provider}&
\textbf{LDTs}& \hline
%
Ericsson &
Enhanced Cell-Id, A-GPS &
%
Openwave &
Cell-Id, AOA, E-OTD, TDOA, A-GPS &
%
Qualcomm &
Enhanced Cell-Id, A-GPS &
%
Cambridge Positioning System &
Cell-Id, OTDOA, GPS &
%
True Position &
Cell-Id, AOA, U-TDOA, A-GPS, Hybrid & \hline
%
\end{tabular}
\label{comparisonSolutions}
\end{table}

\section{Summary}
Starting with today's LDTs this chapter discusses the differences between terminal-based and network-based LDTs. Furthermore it shows the accuracy as well as advantages and disadvantages of LDTs. To summarise the main facts, A-GPS delivers the best results concerning terminal-based LDTs. Cell-Id is the most elementary method, inexpensive and inconsistent because of the diversity in cell size. E-OTD and UL-TOA requires significant infrastructure changes, but  yields better results than Cell-Id. Finally cutting-edge infrastructure providers are presented to give an overview of today's solutions.